The present invention relates generally to a recording apparatus, more specifically to a line scan recording apparatus.
Various types of recording apparatus have been widely used for making hard copies of images. In one type, the recording medium, typically light sensitive paper, perpendicularly traverses a marking means in the form of an intensity modulated deflectable beam, such as the electron beam of a cathode ray tube (CRT), thereby producing on the recording medium a recorded gray scale image representing the input data supplied to the apparatus. One example of the type known as a line scan recording apparatus is that using a fiber optic CRT (FOT). The recording medium is a light-sensitive sheet which is moved across and in contact with the fiber optics strip of the FOT by a sheet drive means. The electron beam of the FOT is moved back and forth along the fiber optics strip while being intensity modulated in accordance with the input data supplied to the apparatus to produce the recorded image.
In such recording apparatus, quality and fidelity of the recorded images depend on the uniformity in velocity at which the recording medium travels. However, short term recording medium velocity fluctuations are unavoidable due to mechanical and electrical characteristics of the recording medium drive means. In order to compensate for the image density distortion due to the short term velocity fluctuations, U.S. Pat. No. 4,172,259 discloses a technique in which the beam of the FOT is deflected vertically in response to the differences in the recording medium velocity by using velocity sensing means, a frequency to voltage converter, a differentiator, an integrator and vertical deflection means to raise or lower the horizontal sweep, for a higher or lower than normal recording medium velocity, respectively.
A predominant factor of the recording medium velocity variation is known as "cogging" which is traceable to imperfections in the motor gear box. As a result of cogging, the recorded image includes what appears to be a repeating light-dark bar pattern superimposed upon the recorded image. This light-dark bar pattern reoccurs with relatively constant spacing therebetween. This spacing is constant regardless of the medium driving speed, indicating its origin is in the drive mechanism. Through measurements, it has been observed that the bar pattern repeats once-per-motor-revolution.
Probably the largest contributing factor to cogging is the misalignment of the worm gear affixed to the shaft of the driving motor and the radial gear on the recording medium transport sub-system. This misalignment can take two forms. The first is a skew between the worm gear and the radial gear. This can result from any of a combination of factors including a bent motor shaft or a gear mounting hole through either the worm gear or the radial gear which is not parallel to the central axis of that gear. The second is an eccentric gear mounting hole through either of the gears. An eccentric mounting hole is one that is parallel to the central axis of the gear, but off center. The normal skew and eccentric alignment of the worm and radial gears is illustrated in FIG. 1.
FIG. 2 illustrates the typical variations of the velocity of the recording medium with V.sub.o being representative of the average value of that velocity. The velocity varies non-sinusoidally as a result of the non-linear way that the pressure between the worm and radial gears builds and releases. The result on the recorded gray-scale image on the recording medium is dark bars when the medium velocity is slowest and light bars when the medium velocity is greatest. The dark bars are caused by the image lines being recorded closer together than those recorded at a slower medium velocity and the light bars are caused by the image lines being recorded farther apart than those recorded at a faster medium velocity. The dark bars have been observed to occur with a 50% duty cycle due to the non-linearities of the light sensitive recording medium.
When using a light sensitive medium to record an image the intensity of the beam exposing the medium is selected to result in interline fill, or bleed, with nominal spacing between the lines of the image to give the total image a continuous appearance. When the line spacing is other than nominal, the individual image lines will either overlap causing a dark bar, or be spaced too far apart resulting in insufficient bleeding between the lines causing light bars or a non-continuous or stretched portion of the total image. A compensation means to reduce or increase the intensity of the beam when the velocity of the recording medium is slower or faster than nominal, respectively, is desirable. Such a device would reduce the over bleeding between close image lines and increase the bleeding between distant image lines. The present invention is believed to provide such a compensation device.